1. Field of the Invention
The present invention relates to a thin film transistor, an active matrix substrate, and a manufacturing method thereof.
2. Description of Related Art
A TFT active matrix substrate (which is hereinafter referred to simply as “TFT substrate”) using a thin film transistor (hereinafter referred to as “TFT”) as a switching element is an example of semiconductor devices. The TFT substrate is used for an electro-optical device such as a display device. The semiconductor device such as TFT has features of low power consumption and low profile. Taking advantage of such features of the semiconductor device, its application to a flat panel display as an alternative to a cathode ray tube (CRT) is increasing.
There is a demand for cost reduction for the semiconductor device. For this reason, a back-channel TFT is widely used for a switching element of a TFT substrate for liquid crystal displays. In the back-channel TFT for liquid crystal displays, amorphous silicon (Si) is typically used as a semiconductor active layer, and an inverted staggered structure is adopted.
Further, development of a TFT using an oxide semiconductor for an active layer has been growing active recently (cf. Japanese Unexamined Patent Application Publications Nos. 2007-284342 and 2007-73702). The oxide semiconductor has a higher mobility than amorphous silicon used hitherto. As the oxide semiconductor, zinc oxide (ZnO) or IGZO containing gallium oxide (Ga2O3) and indium oxide (In2O3) in addition to zinc oxide (ZnO) is mainly used. This technique is disclosed also in Japanese Unexamined Patent Application Publication No. 2005-77822, Nature Vol. 432(2004) p. 488, and Japanese Unexamined Patent Application Publication No. 2007-115902, for example. An oxide semiconductor film has a high translucency. For example, the technique of applying an oxide semiconductor film having a transmittance of 70% or higher for visible light of 400 nm to 800 nm is disclosed in Japanese Unexamined Patent Application Publication No. 2007-115902.
The above-described oxide semiconductor film is insoluble in an alkaline developer for photoresist, just like an oxide conductive film such as ITO (indium oxide (In2O3)+tin oxide (SnO2)) and IZO (indium oxide (In2O3)+zinc oxide (ZnO)), which are known as a transparent conductive film. In addition, the oxide semiconductor film can be etched using a weak acid solution such as oxalic acid or carboxylic acid. Thus, the oxide semiconductor film has an advantage that patterning is easy.
On the other hand, the oxide semiconductor film easily solves in an acid solution which is commonly used when etching a general metal film (Cr, Ti, Mo, Ta, Al and an alloy of them) used for a source electrode and a drain electrode of a TFT. Therefore, when manufacturing a TFT using an oxide semiconductor for an active layer, the technique disclosed in Japanese Unexamined Patent Application Publication No. 2008-72011, for example, is used. According to the technique, the type of a metal film, an etching solution of a metal film, and an oxide semiconductor film is appropriately selected for manufacture. This enables selective etching by which only the metal film for the source electrode and the drain electrode is etched and the oxide semiconductor film remains unetched.
To manufacture a TFT substrate, a metal film and a semiconductor film are sequentially formed. Then, after a photoresist pattern is formed in a photolithography process, etching is performed. A wiring, electrode and semiconductor pattern is thereby formed. In this way, a TFT substrate is manufactured.
When there is an anomaly in the pattern of wiring, electrode or semiconductor, a failure occurs in a performance test after the process is completed, resulting in a decrease in yield. To avoid this, inspection of the final shape of the pattern is performed as appropriate after each process of manufacture in some cases. Namely, it is important for cost reduction to detect a pattern failure in the early stage before completion of products.
The detection of a pattern failure is typically performed by observing the pattern using an optical microscope and inspecting the pattern shape. The inspection of the pattern shape may be performed by automatically reading image data and comparing it with normal shape data or by direct visual examination.
However, in the case of inspecting the pattern shape using the image through the optical microscope, the following problem occurs. Specifically, the inspection of a material with high transmittance like the oxide semiconductor film is difficult compared to a material that reflects light like the metal film, causing a decrease in detection sensitivity. This leads to a decrease in productivity.
Further, as described above, in the case of using a known typical oxide semiconductor film, selective etching with a known typical metal film as a wiring and electrode material using a chemical solution is difficult. Accordingly, the variety of combinations of a metal film, a semiconductor film and an etching chemical solution is limited. This raises an issue that the range of choice of materials in improvement of TFT characteristics or the like is significantly restricted.
The present invention has been accomplished in view of the above circumstances, and it is desirable to provide a thin film transistor allowing a broad range of choice of materials and having high productivity, an active matrix substrate, and a manufacturing method thereof.